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1. Field of the Invention
The invention herein relates to a method and a system to reducing the viscosity of crude oil at the earth""s surface to improve the flowability and pumpability and to thereby augment the movement of crude oil through pipelines and process equipment. The invention herein is intended to achieve the ultimate goal of reducing the problems and expense of moving heavier or more viscous crude oil through pipeline and process equipment.
2. Background of the Invention
Others have taught the concept of mixing gases with liquids to change the characteristic of the liquids, including the viscosity thereof. More particularly, others have suggested the use of gases mixed with crude oil, particularly in subterranean locations, to augment production of crude oil. As an example, U.S. Pat. No. 5,025,863 entitled, xe2x80x9cEnhanced Liquid Hydrocarbon Recovery Processxe2x80x9d teaches the use of natural gas injected into an oil bearing formation to render the liquid hydrocarbons mobile and thereafter the mobilized liquid hydrocarbons are more easily produced from the well.
In U.S. Pat. No. 5,104,516 entitled, xe2x80x9cUpgrading Oil Emulsions With Carbon Monoxide or Synthetic Gasxe2x80x9d involves contacting a water emulsion of a heavy oil with carbon monoxide at a temperature range such that a water gas shift reaction takes place to thereby assist in reducing the viscosity of the crude oil.
For other background information relating to the general subject matter of the invention herein reference may be had to the following United States patents:
Crude oil, particularly that produced from certain geological formations, can be relatively viscid, that is, can have a viscosity that makes it difficult to transfer through a pipeline. Heavy, thick viscid crude oil is referred to in the petroleum industry as xe2x80x9clow gravityxe2x80x9d oil; high gravity crude oil being that which is relatively thin and relatively easy to pump.
The viscosity of crude oil is affected by temperature and one way to decrease the viscosity of crude oil is to increase the temperature. While increasing temperature is a common way to reduce viscosity it is an expensive procedure and is not an acceptable procedure for improving the viscosity of crude oil that must be transmitted over a relatively long distance pipeline since maintaining an elevated temperature of crude oil in a pipeline is extremely difficult.
Another way of decreasing the viscosity of crude oil is to mix with it an immiscible high gravity liquid component. For instance, gasoline, kerosene or other high gravity components can be mixed with viscid crude oil to reduce the viscosity so that it can be more effectively pumped. At the destination, the added gasoline, kerosene or so forth can be removed and recycled. This procedure works effectively to reduce the viscosity of crude oil but is expensive and in many applications impractical, particularly where crude oil must be pumped over a relatively long distance so that thereby recirculating the thinning agent becomes a serious problem.
Viscosity is the degree to which a fluid resists flow under an applied force. Viscosity is measured by the tangential stress on the fluid divided by the resultant viscosity gradient under conditions of streamlined flow. The unit of measurement of viscosity is xe2x80x9cpoisexe2x80x9d. Poise is a centimeter-gram-second unit of dynamic viscosity equal to one dyne-second per square centimeter. Viscosity is usually expressed in 1/100th of a poise, that is, in centipoise. In the petroleum industry the pumpability of crude oil is usually characterized by its gravity. High gravity crude oil is thin and easily pumpable. Low gravity crude oil is thick and difficult to pump. High gravity equates to low viscosity and high viscosity to low gravity.
The present invention is concerned with a method of decreasing the viscosity of crude oil in a manner that does not require elevating its temperature or the use of a high gravity liquid thinning component. The present invention achieves reduced viscosity of crude oil by injecting into the crude oil, under high pressure, a gas, or a combination of gases. Particularly the invention is concerned with injecting into crude oil carbon dioxide (CO2) or more preferably, a combination of CO2 and nitrogen (N).
In a system for practicing the invention, CO2 is thoroughly admixed with crude oil at an elevated pressure. Instead of CO2 only, a mixture of CO2 and N may be employed in ratios ranging from 8% N, 92% CO2 to 92% N, 8% CO2. The CO2, or mixture of CO2 and N, is thoroughly admixed with crude oil at a pressure of at least about 600 lbs per square inch (psi). A preferred pressure for admixing crude oil and gas to achieve decreased viscosity is from about 600 psi to 1800 psi although the maximum upper pressure is limited only by the availability of equipment and the expense of attaining the higher pressure.
In practicing the invention the crude oil is introduced into a mixer that may be in the form of an elongated horizontal cylindrical treating vessel having within it a reduced diameter centralized gas injection pipe, the pipe having a plurality of spaced apart small diameter openings therein. A spiraled, auger-shaped fin is affixed to the gas injection pipe to cause the crude oil flowing through the treating vessel to take a circuitous route and to thereby cause a more thorough admixing of injected gas and crude oil.
The quantity of gas employed is determined by the viscosity reduction required. For maximum viscosity reduction the maximum gas the crude oil will absorb is used. Stated another way, the reduction in viscosity is most effectively and efficiently obtained by employing gas the rate at which, for the treating pressure level, all of the gas is absorbed by the crude oil.
Experience has indicated that crude oil, having been treated to cause the absorption of CO2 or a combination of CO2 and N, at elevated pressures, attains a reduced viscosity that is relatively long lasting. When the treated oil is exposed to ambient pressure the dissolved gas eventually separates out of solution and the crude oil will eventually revert to its natural viscosity, however, the rate of separation is not instantaneous when pressure reduction occurs but is a relatively slow process so that crude oil, after having the viscosity reduced by the methods of this invention can be pumped efficiently over relatively long distances.